Field of the Invention
The present invention relates to an optical film that is useful for a surface protective film for an image display device, and the like, and to a polarizing plate and an image display device that have the optical film.
Background Art
A cellulose acylate film, which is represented by cellulose acetate, has high transparency and has been used for various purposes in an image display device. For example, a cellulose acylate film is used as a polarizing plate protective film in a liquid crystal display device since the adhesiveness thereof to polyvinyl alcohol used in a polarizer may be easily secured.
In recent years, a liquid crystal display device, particularly a liquid crystal display device for a middle sized or small sized equipment, undergoes drastic reduction in thickness, and thus reduction in thickness of members used therein, particularly reduction in thickness of a polarizing plate protective film (such as a protective film having a hard coat layer provided on a surface of an image display device, a protective film functioning as a retardation film, and an ordinary protective film having a small phase difference), is being demanded.
As a protective film provided on a surface of an image display device, which is represented by a liquid crystal display (LCD), a plasma display panel (PDP) and an electroluminescent display (ELD), a cellulose acylate film having provided thereon a hard coat layer for imparting physical strength, such as scratch resistance, thereto is generally used. The protective film is generally disposed outside the polarizer, particularly outside the polarizer on the viewing side.
Under the circumstances, Patent Document 1 describes that an ester oligomer is added to a thin cellulose acylate film having a thickness of 20 μm, 40 μm, 60 μm or the like to achieve a retardation film having a high Re value (in which Re shows the retardation in the in-plane direction, hereinafter the same) or a high Rth value (in which Rth shows the retardation in the thickness direction, hereinafter the same), and the application thereof to a liquid crystal display device may prevent color unevenness on displaying an black image due to temperature fluctuation and light leakage occurring on the outer periphery of the display part after the lapse of time in a high temperature and high humidity environment, and may reduce the bleed out.
Patent Documents 2 to 5 describe a cellulose acylate film having an aromatic ring-containing ester oligomer added thereto. Patent Document 2 describes that an aromatic ring-containing ester oligomer may impart a high retardation value to a film formed of a cellulose ester resin, and the example thereof describes a cellulose ester film with a thickness of 80 μm having a high Rth value.
Patent Document 3 describes that the addition of an aromatic ring-containing ester oligomer may improve the moisture permeation resistance of a cellulose ester film and may improve the bleed resistance under a high temperature and a high humidity. The example thereof describes a cellulose ester film with a thickness of 80 μm having a high Rth value.
Patent Document 4 describes that the addition of an aromatic ring-containing ester oligomer may suppress fluctuation of the retardation value of a cellulose acylate film depending on the humidity. The example thereof describes a cellulose acylate film using an aromatic ring-containing ester oligomer containing phthalic acid, isophthalic acid or terephthalic acid as an aromatic dicarboxylic acid component, and describes a retardation film with a thickness of 40 μm having a high Re value and a high Rth value.
Patent Document 5 describes a cellulose acylate film having an aromatic ring-containing ester oligomer added thereto may be suppressed in fluctuation of the retardation depending on the humidity and fluctuation thereof under a hydrothermal environment. The example thereof describes a cellulose acylate film using an aromatic ring-containing ester oligomer containing phthalic acid or isophthalic acid as an aromatic dicarboxylic acid component, and also describes a retardation film with a thickness of from 10 to 50 μm having a high Re value and a high Rth value.